1. Field of the Invention
This invention pertains to circuitry which can be used to extract a transmission clock signal from isochronous modulated transmissions (hereinafter this line-signal is referred to as data transmissions) in order to facilitate subsequent use of the data contained in such transmissions.
2. Description of the Prior Art
Because of the increased amount of information which can be transmitted through telephone data lines, it is now common practice to modulate a carrier in order to transmit such information within a telephone network. In such applications, use is commonly made of modems, which are modulator-demodulator circuits. When such apparatus is used, information actually transmitted through the telephone network is transmitted in modulated data transmissions wherein the modulating waveform is a binary square-wave data signal.
Various conversion techniques may be utilized by such modems. For transmission rates of less than 1200 bits per second, methods using frequency-shift keying are commonly employed. In modulation techniques of this kind, a negative condition in the underlying data signal is assigned a symbol of 1300 Hz in the modulated data transmission, and a positive condition is correspondingly assigned a symbol of 2100 Hz. In such a system, only two different symbols are utilized to transmit the information.
However, when transmission speeds of greater than 1200 bits per second in the modulated data transmission are employed, it is necessary to utilize more than two symbols in order to achieve acceptable data accuracy at such high data transmission speeds. In general, the number M of symbols will be equal to 2.sup.N, wherein N is the number of bits. In fact, if the rate R of transmission speed is considered, it will appear that R will be equal to N multiplied by F.sub.T, wherein F.sub.T is equal to the frequency of the transmission clock of the data transmission.
For such transmission speeds in excess of 1200 bits per second, differential phase-shift keying (hereinafter referred to as DPSK) techniques are used in order to modulate a carrier with the underlying data signal. In DPSK techniques, the phase of the carrier is sampled in phase with the clock frequency F.sub.T of the modulated data transmission, and each difference in phase between two adjacent sample points constitutes the symbol for data present in the carrier.
Many reasons exist why the extraction of a clock signal from a data transmission is desirable. Firstly, it is helpful to have this transmission clock signal available at a receiving station in order to utilize it to demodulate the modulated data transmission. Secondly, data-processing equipment may be present at the receiving end of a telephone system, as in the case of a computer which is connected to a remote terminal by means of a telephone line. In cases such as this one, it is advantageous to derive the speed (or frequency) with which data is actually transmitted. As mentioned above, this speed can be determined in the event that the number of bits utilized in the DPSK system and the clock frequency of the data transmissions are known.
Those skilled in the art are aware that various techniques are available in order to derive the frequency of the clock signal of the underlying data signal from the frequency of the transmission clock signal of the modulated data transmission. For example, it is possible to utilize frequency multiplication under the influence of phase-controlled attenuation to do this. The main difficulty with using techniques of this type is the original extraction of the transmission clock signal from the PCM data transmission itself.
In a publication entitled "Ein Vorschlag zur Taktsynchronisation bei Datenubertragung" by Joachim Swoboda, Archiv der Elektrischen Ubertragung, Volume 22, Part 11 (November 1968) pp. 509-513, methods of recovery of the clock signal of an underlying data signal from the changes in state of that signal are described. These changes in state occur at spacings of one clock pulse of the data signal or at spacings of an integral multiple of the duration of one such clock pulse. Hence, such changes in state can be used to readjust the frequency of an oscillator in a phase-controlled circuit in a receiver that receives the signal. The frequency of this oscillator may be adjusted either continuously or in steps, the latter alternative being accomplished by means of a switchable frequency divider.
However, such a system has the disadvantage that the clock signal of the modulating data signal which is impressed upon the modulated data transmission cannot be obtained directly from the carrier. Rather, a complicated demodulating circuit must be used. However, it is also difficult to demodulate the modulated data transmission with the aid of its own transmission clock signal if this transmission clock signal must first be obtained from the underlying data signal which is produced at the demodulator output.
It would therefore be advantageous to provide a circuit which would enable a transmission clock signal to be simply and directly extracted from the data transmission itself, without the intermediary of a complicated demodulator.